This invention relates to a grinding apparatus for grinding a stamper used for molding a signal recording part of an information recording disc. More particularly, it relates to a grinding apparatus for grinding a reverse surface of the stamper carrying projections and recesses on its front surface for molding similar projections and recesses as pits and lands corresponding to the information signals.
There has hitherto been proposed an optical disc having a disk-shaped disc substrate on one major surface of which pits and lands corresponding to information signals are formed. The disc substrate of the optical disc is formed of light-transmitting synthetic resin, such as polycarbonate or polymethyl methacrylate. A reflecting film of e.g. aluminum is deposited on the major surface of the disc substrate, such as by vapor deposition or sputtering.
With such optical disc, the information signals are read by detecting the state of the pits and lands from the opposite major surface of the disc by optical means.
For molding the disc substrate, a stamper or master disc, having projections and recesses on its major surface, is employed. The stamper is a disk of nickel or a like metal having a diameter substantially equal to the diameter of the disc substrate. The projections and recesses formed on the major surface of the stamper correspond to the information signals formed as pits and lands on the stamper. However, the state of the projections and recesses on the substrate is reversed from that of the projections and recesses on the stamper.
The reverse surface of the stamper is a planar surface. The stamper is mounted on a planar base block in a metal mold unit in an injection molding apparatus so that the reverse surface is set On and supported by the base block. During the process of molding the disc substrate, the stamper is kept in pressure contact with the base block by an injection pressure of a material which forms the disc substrate.
If the reverse surface, the mounting surface, of the stamper has irregularities, the stamper is deformed to conform to the irregularities of the mounting surface so as to render it impossible to effect correct molding. For this reason, %he stamper mounting surface is ground to a smooth mirror surface by a stamper grinding apparatus.
The conventional stamper grinding apparatus has a turntable on which the stamper is set. The apparatus is rotated with the stamper held thereon. The stamper is held to the turntable with its mounting surface directed upwards.
The stamper grinding apparatus includes an abrasive member which is kept in sliding contact with the mounting surface of the turntable-mounted stamper. The abrasive member comprises an abrasive tape having hard fine grains deposited on a film-shaped substrate and a supporting member supporting the abrasive tape. An abrasive surface of the abrasive tape contacts the mounting surface of the stamper with a predetermined contact pressure. The abrasive member is supported for movement in a direction towards and away from the center of rotation of the turntable and in a direction parallel to the mounting surface of the stamper held on the turntable, that is in a direction spanning the inner and outer peripheries of the stamper.
With the above-described stamper grinding apparatus, when the stamper is rotated with the abrasive member contacting the mounting surface of the stamper, the abrasive member is brought into sliding contact with the mounting surface of the stamper for grinding the mounting surface. The abrasive member is moved across the inner and the outer peripheries of the stamper for grinding the mounting surface of the stamper in its entirety.
Meanwhile, with the above-described stamper grinding apparatus, the turntable is always rotated at a substantially constant angular velocity. Thus the relative speed of sliding contact between the abrasive member and the stamper is gradually changed across the inner and outer peripheries of the stamper.
Consequently, the amount of grinding per unit time, and hence the amount of wear caused to the abrasive tape differs between the inner and outer peripheries of the stamper. Thus, it is difficult with the above-described stamper grinding apparatus to grind the disc substrate across its inner and outer peripheries under a constant state. That is, if the amount of grinding is to be constant across the inner and outer peripheries of the stamper, it becomes necessary to change the grinding time for the inner periphery of the stamper from that for the outer periphery of the stamper such that it is difficult to grind the stamper to a desired uniform smoothness.
On the other hand, with the above-described stamper grinding apparatus, since the amount of wear caused to the abrasive tape per unit time differs at the inner and outer peripheries of the stamper, the grinding state is changed across the inner and outer peripheries of the stamper if the feed rate of the abrasive tape is kept constant.
Besides, since the abrasive tape used in the above-described stamper grinding apparatus is worn out with the progress in the grinding of the stamper, the abrasive tape is fed so that its used-up portion is taken up and its unused portion is kept in sliding contact with the stamper during grinding of the mounting surface of the stamper.
Another conventional practice in the stamper grinding apparatus has been to wobble the abrasive member radially of the stamper with a predetermined frequency and a predetermined small amplitude for increasing an area on the stamper that may be ground with the abrasive tape of a unit length, that is, for raising the grinding efficiency of the stamper. If the abrasive member is wobbled in this manner, the locus of movement of the abrasive member on the stamper is wobbled in the form Of waves meandering radially inwardly and outwardly of an arc of a circle having the center of the stamper as its center.
However, if the abrasive member is wobbled radially of the stamper in the above-described stamper grinding apparatus, true grinding apparatus must be structurally complicated.
On the other hand, if the abrasive member has a high weight, it is difficult to wobble the abrasive member at a high frequency, so that not only must the apparatus be complicated in structure but the grinding process cannot be executed speedily.
Above all , in a stamper grinding apparatus having plural grinding members (for rough finishing, semi-finishing and finishing), it is necessary to provide a wobbling mechanism for each of the abrasive members, thereby complicating the structure further. Besides, with the stamper grinding apparatus having plural abrasive members, it is difficult to provide for equal wobbling frequency and amplitude of the respective abrasive members, thus rendering it difficult to realize satisfactory grinding.